The present invention generally relates to the medical condition of obstructive sleep apnea, and relates in particular to systems and methods for detecting obstructive sleep apnea in the awake state.
Obstructive sleep apnea (OSA) is caused by the collapse of the tongue and soft palate onto the posterior pharyngeal wall, which obstructs the airway intermittently during sleep. Loud snoring, in combination with obesity, is known to be highly predictive of OSA. Symptoms of OSA are known to include pauses or reduction in breathing during sleep, having an unrefreshed feeling after sleep, and excessive daytime sleepiness. OSA has been reported to be associated with serious health consequences due to the increasing risk of cardiovascular disease, stroke, hypertension, arrhythmias, diabetes, and sleep deprived driving accidents. The prevalence of sleep apnea is reported to be not well defined due to an expected high level of under diagnosis; it has been estimated that about 20% of the adults in the United States have OSA, of whom only about 10% have been diagnosed.
The conventional diagnosis of OSA relies on testing done during an overnight sleep study using polysomnography. A value referred to as the apnea hypopnea index (AHI) is the average number of apneas and hypopneas per hour of sleep determined from the polysomnographic study. The AHI index values have been used to classify OSA as mild (AHI=5-15), moderate (AHI=15-30), and severe (AHI>30). While apnea is defined as the cessation of airflow for more than 10 seconds, the definition of hypopnea is yet to be standardized. In addition to the original (Chicago) definition of hypopnea that requires either >50% airflow reduction or a lesser airflow reduction with associated >3% oxygen desaturation or arousal, two other stricter definitions have been used by others. The overnight polysomnographic study is highly specialized, expensive and time consuming, which has contributed in part to the under diagnosis of OSA.
Several simplified portable devices have been developed to facilitate unattended home-based sleep studies for the diagnosis of OSA. These devices may typically be used at the individual's own home, thereby obviating inconvenience and the high cost of using a sleep laboratory study. A guideline established by the Portable Monitoring Task Force of the American Academy of Sleep Medicine suggests that such devices can be used in patients with a high pretest probability of moderate to severe OSA, but are not appropriate for general screening of asymptomatic populations (see Clinical guidelines for the use of unattended portable monitors in the diagnosis of obstructive sleep apnea in adult patients, by N. A. Collop, W. M. Anderson, B. Boehlecke, D. Claman, R. Goldberg, D. J. Gottlieb, D. Hudgel, M. Sateia and R. Schwab, J. Clinical Medicine, v. 3, no. 7, pp. 737-747 (2007)). A 2010 review of the effectiveness of home-based sleep studies concludes that more outcome-oriented studies are needed to resolve the controversy surrounding the appropriateness of home-based sleep studies (see Are sleep studies appropriately done in the home?, by P. D. Gay and P. A. Selecky, Respiratory Care, v. 55, no. 1, pp. 66-75 (2010)).
The article Rapid screening test for sleep apnea using a non-linear and non-stationary signal processing technique, by J. Salisbury and Y. Sun (the present inventors), Medical Engineering and Physics, v. 29, no. 2, pp. 150-157 (2007), discloses a method to detect OSA from a 5-minute daytime recording of the nasal airway pressure. Due to the non-linear and non-stationary nature of the signal, the Hilbert-Huang transform (see The empirical mode decomposition and the Hilbert spectrum for non-linear and non-stationaty time series analysis, by N. E. Huang, Z. Shen, S. R. Long, M. C. Chu, H. H. Shih and A. Zheng, Proceedings of the Royal Society of London, Series A, no. 454, pp. 903-995 (1998)) was used to extract signals intrinsic to OSA. The Hilbert spectrum was centered around 1.5 Hz for normal subjects and shifted upward in frequency scale with increased likelihood of OSA. Although this study did find that it was possible to obtain a marker for OSA from data collected in the waking state, the methodology has not been fully successful in accurately identifying OSA conditions in all subjects and requires extensive computation.
The article Posture-Dependent Change of Tracheal Sounds at Standardized Flows in Patients With Obstructive Sleep Apnea, by H. Pasterkamp, J. Schafer and G. Wodicka, American College of Chest Physicians, v. 110, no. 6, pp. 1493-1498 (1996), discloses performing measurements of the tracheal sound intensity (TSI) from a subject in order to detect increased sound levels when a patient is in a supine position, thereby indicating increased flow resistance in the trachea. The analysis of the data involved determining an average power spectrum of tracheal sounds within low (0.2 to 1 KHz), medium (1 to 2 KHz), and high (2 to 3 KHz) frequency bands for each subject. The study found that at the same inspiratory flow, the increase in tracheal sound intensity from upright to supine position was greater in OSA patients than in control subjects. The control subjects, however, were significantly younger, not obese and did not include snorers. There remains a need, therefore, for an efficient and effective sleep apnea test system and method that may readily detect sleep apnea for a subject in a waking state during daytime in the office of a primary-care physician.